Regulation of the cell cycle is highly conserved among eukaryotes, and many of the genes involved have been isolated from wide range of organisms such as yeasts, animals and plants. In mammalian cells, progression through the G1/S transition is controlled by cyclin D in conjunction with their catalytic partners, CDK4 and CDK6. The essential role of cyclin D/CDK complex is to direct phosphorylation of pRb, thereby driving cells through the restriction point at late Gi phase.We have isolated four cDNA clones for cyclin D (NtcycD3), which all belong to the CycD3 class, Using the highly synchronized suspension culture of tobacco BY-2 cells , we analyzed the transcript levels of these genes during the cell cycle. In contrast to mammalian cyclin D, NtcycD3 showed various expression patterns in the cell cycle.Mammalian cyclin D can form active complexes with CDK4 and CDK6, but not with Cdc2. Although a number of cdc2-related and cyclin genes have been isolated in plants, it is not elucidated which combination of these proteins forms active complexes. We have produced NtcycD3 and tobacco Cdc2 (Cdc2Nt1) with a baculovirus expression system and found that these proteins formed a complex. Surprisingly, the complex can phosphorylate the tobacco Rb-related protein in vitro.